This invention relates to an equalizer with a variable equalizing time constant which is changed according to the input signal level, and more particularly, to a playback equalizer for a tape recorder having a variable equalizing time constant wherein the higher frequency response is emphasized with reduction of the recording level.
Recent home use cassette decks mostly include a built-in noise reduction system. For such noise reduction system, a B-type Dolby system having an encoding characteristics as shown in FIG. 1 has been extensively used. This system is extremely effective for reducing the tape hiss noise, and Dolby encoding is widely adopted in the cassette tape recording. Most cassette tapes adopt Dolby encoding whether they are recorded by the user or commercially available music cassette tapes. For obtaining proper playback of such Dolby encoded recording tape, a correctly adjusted regular Dolby decoder is necessary. The circuitry of such a decoder is disclosed in, for instance, FIG. 4 of U.S. Pat. No. 3,631,365 (Dec. 28, 1971, Ray M. Dolby). However, the regular Dolby decoder is complicated in circuit construction and increases in no small quantities the cost of manufacture. Even if an exclusive IC including a Dolby decoder is used, many peripheral circuit component parts involved constitute a problem when it is intended to reduce the size of the product.
This problem becomes more serious where a noise reduction system such as the Dolby system is incorporated in a microcassette. The tape running speed of the microcassette (typically being 2.4 or 1.2 cm/sec) is slow compared to the Philips type cassette. Therefore, incorporation of a noise reduction system is desired from the standpoint of improving the signal-to-noise (S/N) ratio. However, for a microcassette tape recorder, particularly for a playback only type portable microcassette tape recorder, small size, light weight and low price are desired. Therefore, it is often infeasible to assemble a regular noise reduction decoder such as the B-type Dolby decoder in a microcassette tape player. For playing back a Dolby encoded tape without use of any regular decoder, a high frequency cutting by a tone control circuit has been known as a convenient method. With this method, however, when a high level signal (usually above -10 to -20 VU in level in the case of FIG. 1) with a small preemphasis is reproduced at the time of playback, too much attenuation over the high frequency range results, thus considerably deteriorating the quality of the reproduced sound.